Note blood group, regarding blood (4)
I believe the answer is:
gore
'blood' is the definition.
('gore' can be a synonym of 'blood')
'note blood group regarding' is the wordplay.
'note' becomes 'g' (musical note).
'blood group' becomes 'o' (O is an example**).
'regarding' becomes 're'.
'g'+'o'+'re'='GORE'
(Other definitions for gore that I've seen before include "Spilt blood" , "piece of material" , "Clotted blood" , "Showing guts" , "Pierce with horn - blood" .)